This proposal has three specific aims, which probe three different sets of biologically relevant electron transfer reactions in proteins. These electron transfer reactions involve redox-active amino acid residues and chlorophyll (chl). We will test three hypotheses: (A) that tyrosyl radical spin density delocalizes into the peptide/protein amide bond in a conformationaUy-sensitive, sequence-dependent manner; (13) that electron transfer-linked proton transfer distinguishes the redox-active tyrosines in photosystem II (PSID; and (C) that a difference in chlorophyll tautomerization state contributes to the difference in midpoint potential between the primary chl donors in PSII and photosystem I (PSI). To address these questions, electron paramagnetic resonance spectroscopy, Fourier-transform infrared spectroscopy, and transient kinetics will be used. In collaborative efforts, electron spin-echo envelope modulation and density functional theory will also be employed. Long distance electron transfer in proteins involves step-wise electron transfer reactions between pairs of redox-active prosthetic groups, which act as catalytic intermediates. These prosthetic groups include covalently and non-covalently bound cofactors, such as heme, pheophytin, and chl, as well as amino acid side chains. An important long-term goal of this research project is to determine how electron transfer rates are influenced by changes in the structure and environment of these redox intermediates. The overall goal of this proposal is to elucidate the structural and environmental factors that are important in the control of midpoint potential and electron transfer in PSII and PSI. The spectroscopic studies to be described here will give new information concerning electron transfer mechanisms in photosynthetic reaction centers. The factors that influence the efficiency of electron transfer reactions are likely to be important in other enzymatic redox reactions, and photosynthetic proteins provide a unique and useful model system, because the reactions can be controlled by light.